1. Field of the Invention
This invention relates to a control unit for a spark ignition-type engine, more specifically, a control unit for controlling the state of combustion in each cylinder of a multi-cylinder engine which is capable of improving fuel efficiency and exhaust emissions.
2. Description of the Related Art
In a conventionally-known spark ignition-type engine which is capable of improving fuel efficiency, combustion is conducted with the air-fuel ratio of a mixture inside each cylinder being kept at a lean air-fuel ratio higher than a stoichiometric ratio. Particularly, in an engine which is provided with a fuel injection valve for injecting fuel directly into a combustion chamber, stratified charge combustion is conducted by injecting fuel from the fuel injection valve in a lower speed & lower-load range or the like at a compression stroke, leading to the realization of a super-lean burn (for example, refer to Japanese Patent Laid-Open No. 10-29836).
In such an engine, however, only an ordinary three-way catalyst used to purify exhaust gas (having a high capability to remove HC, CO and NOx at and around a stoichiometric ratio) is inadequate to remove NOx at the time of a lean operation. As shown in Japanese Patent Laid-Open No. 10-29836, therefore, a lean NOx catalyst is provided which absorbs NOx in an oxygen-excess atmosphere and removes and reduces NOx in a lowered oxygen-concentration atmosphere. In the case where such a lean NOx catalyst is used, when the quantity of NOx that the lean NOx catalyst absorbs during a lean operation increases, for example, as shown in Japanese Patent Laid-Open No. 10-29836, additional fuel is injected during an expansion stroke, in addition to main combustion. Thereby, the air-fuel ratio of exhaust gas is made rich and CO is formed, promoting the removal and reduction of NOx.
Problems to be Solved by the Invention:
In an engine which conducts such a conventional lean operation as mentioned above, the lean NOx catalyst needs to be provided in an exhaust passage so that its purification capability for NOx can be secured during the lean operation, resulting in a rise in expenses. Furthermore, as mentioned above, when the quantity of NOx that the lean NOx catalyst absorbs increases, NOx has to be removed and reduced so that the purification capability of the lean NOx catalyst for NOx can be maintained. This requires making the air-fuel ratio temporarily rich by supplying additional fuel and other means. Furthermore, if fuel to be used is sulfuric, sulfurous poisoning has to be eliminated from the lean NOx catalyst. This needs regeneration processing such as heating the catalyst and supplying a reducing agent, causing the fuel-efficiency improving effect to decrease. Furthermore, if the air-fuel ratio of a mixture becomes leaner than a certain level, that makes the burning velocity too slow, hindering combustion shortly before its completion from working normally. As a result, there is a limit to the improvement of fuel efficiency through making the air-fuel ratio lean under stratified charge combustion.
In consideration of the aforementioned problems, the present applicant has filed an application related to a control unit for a spark ignition-type engine which is capable of keeping the good effect of a lean burn on fuel efficiency and improving its exhaust purification capability only by using a three-way catalyst (Japanese Patent Application No. 2002-024548).
The present invention, based on this kind of art, provides a control unit for a spark ignition-type engine which is capable of securing an exhaust purification capability and improving fuel efficiency more effectively.